The invention relates to a control system for a permanent-magnet electrical machine and a method for controlling a permanent-magnet electrical machine as used, for example, in hybrid drive systems.
DE 102 05 963 A1 discloses a method and control system for controlling a permanent-magnet machine, with each phase of the electrical machine having an associated half-bridge arrangement with a first and a second switching element for supplying power via an intermediate circuit. An operating state of a control system is monitored and compared with a threshold value. When the threshold value is exceeded, a fault state is detected and a short circuit is generated between the phases of the electrical machine.
Permanent-magnet electrical machines are used, for example, as vehicle drive motors which, in the case of hybrid drive systems, receive electrical energy from an energy supply means. Energy is supplied, for example, by a battery which is connected via a converter and an intermediate circuit; in the case of fully electrically operated vehicles energy is supplied by a so-called traction battery or by a generator which is driven by an internal combustion engine.
Because of their design, permanent-magnet electrical machines have the fundamental problem that a counter voltage, which is known as magnet wheel voltage is induced in the armature windings due to the relative movement between the armature windings and the permanent magnets during operation. This induced voltage increases as the rotation speed increases and can reach, and even exceed, the supply voltage of the electric machine. If faults occur in such drive systems with permanent-magnet electrical machines, for example, malfunctions of the control electronics providing for field weakening or a short circuit between the windings, electrical energy will be fed back from the electrical machine to the energy supply means. As a result, a braking torque can be generated in an electric machine which has been shut down but is still rotating. This is very undesirable during operation of a vehicle. Such braking torques can also be generated at low rotation speeds, for example by a three-phase short circuit which is initiated when a fault has been detected.
It is the object of the present invention to reduce the braking torque of an electrical machine which has been shut down but is still rotating.